Kaolin is a naturally occurring, relatively fine, white clay which may be generally described as a hydrated aluminum silicate. Kaolin clay, after purification and beneficiation, is widely used as a filler and pigment in various materials, such as rubber and resins, and in various coatings, such as paints and coatings for paper.
Crude kaolin clay, as mined, contains various forms of discoloring impurities, two major impurities being anatase (TiO.sub.2) and iron oxides. To make the clay more acceptable for use in the paper industry, these impurities must be substantially removed by appropriate techniques.
The production of high brightness clays usually includes at least two processing steps. In a first step, a significant portion of the impurities, mainly anatase, is removed by employing one or more physical separation techniques, such as high gradient magnetic separation, froth flotation and/or selective flocculation. In a subsequent step, the remaining impurities, mainly iron oxides, are removed by known techniques, such as chemical leaching.
Froth flotation is regarded as one of the most efficient methods for removing colored impurities from kaolin clay. Typically, clays to be beneficiated by froth flotation are first blunged in the presence of a dispersant and pH modifier and then conditioned with a collector. The job of the collector is to selectively adsorb to impurities and render them hydrophobic. This part of the process is referred to as conditioning. The conditioned impurities, mainly titanium dioxide in the form of iron-rich anatase, are then removed in a flotation machine via the attachment of the hydrophobic impurities to air bubbles which are injected into the feed slurry or into the flotation pulp.
Two general categories of compounds are reported in the literature as collectors for titaniferous impurities in kaolin clay. Cundy U.S. Pat. No. 3,450,257 discloses the use of fatty acid compounds as collectors, and Yoon & Hilderbrand U.S. Pat. No. 4,629,556 discloses the use of hydroxamate compounds as collectors. Each category of compounds has advantages and disadvantages.
One of the advantages of the fatty acids is that, in addition to collecting impurities, they can also act as frothers when the pulp pH is 8.5 or higher. This may obviate the need for an additional frother in the process. A major disadvantage of fatty acids is that, for them to act as collectors, they must first be activated by polyvalent cations such as Ca.sup.+2 and/or Pb.sup.+2. Unfortunately, this activation process is not a very selective one. The activated collector can adsorb not only to the impurities but also to some of the clay particles which are consequently rendered hydrobophic and, therefore, prone to float as if they were impurities. This leads to losses of clay and inefficiencies in the flotation process.
The very high selectivity towards the impurities without needing an activator has made the hydroxamates a feasible alternative as collectors for titaniferous impurities in kaolin clay. The main disadvantage of hydroxamates is their relatively poor frothability (compared to the fatty acids), which makes the hydroxamates difficult to use in a column cell where a deep froth must be sustained; see Yoon et al., Minerals Engineering, Vol. 5, Nos. 3-5, pp. 457-467 (1992). This may necessitate the use of a frother when the separation is conducted in a column cell. The use of a frother with a hydroxamate is a disadvantage for two reasons: a) the reagent addition system is more complicated and b) frothers can cause excessive foam in the flotation product, thereby making further processing difficult and potentially damaging the quality of the final product. The use of an activator and a frother tends to make the flotation process difficult and less adaptable to different types of kaolin clay.
Therefore, a need exists in the kaolin clay industry for a collector system which will selectively adsorb to the titaniferous impurities in kaolin clay and avoid the necessity of additional chemicals (e.g., activators and frothers).